A thermal energy storage system comprises a thermal energy storage fluid contained within a tank. In operation, thermal energy is stored within the fluid by extracting the fluid from the tank, pumping the extracted fluid through a heat exchanger wherein thermal energy in the form of heat is added to the fluid thereby raising its temperature, and returning the heated fluid to the thermal energy storage tank. As is well known in the art, the density of a fluid decreases as the temperature of the fluid increases. As such, a fluid at a relatively lower temperature will have a density which is relatively higher than the density of the same fluid at a relatively higher temperature. Accordingly, the density of the heated fluid returned to the tank will be relatively lower than the density of the fluid extracted from the tank. Therefore, the relative higher temperature and relatively less dense fluid will stay above the relatively lower temperature and relatively more dense fluid. As can be seen, the fluid at the highest temperature will stay near the top of the thermal storage tank while the fluid at the lowest temperature will stay towards the bottom of the tank. This separation of the relatively warmer and relatively less dense fluid from the relatively cooler and relatively more dense fluid is known as thermal stratification or thermocline. In a perfectly stratified thermal storage tank, only the fluid at the highest temperature will be at the top of the tank, and only the fluid at the coldest temperature will be at the bottom of the tank. However, in a typical thermal storage tank, there exists a transition layer between the hot and cold regions.
One shortcoming of thermal energy storage systems is that it is relatively difficult to maintain a thermocline in a fluid within a thermal storage tank. The process of repeatedly extracting the fluid at a relatively cooler temperature from near the bottom of the tank and returning the fluid at a relatively higher temperature to near the top of the tank increases the width of the transition layer resulting in a degradation of the thermocline in the fluid within the thermal storage tank. The ratio of the difference between the height of the fluid in the tank and the width of the transition layer to the total height of the fluid in the tank is known in the art as the utilization factor. The utilization factor provides an indication of the amount of useful thermal energy stored in the tank. Accordingly, as the width of the transition layer increases, the utilization factor, and therefore the thermal storage capacity, decreases.
As is well known in the art, the thermocline in a thermal storage tank degrades due to a number of factors such as conduction between the layers of fluid at different temperatures, mixing of the fluid due to turbulence, an increase in the velocity of the fluid, etc. Additionally, thermocline degradation becomes more severe where there is an insufficient amount of thermal energy available. The repeated process of partially charging the tank, i.e., adding thermal energy to the tank, and not completely discharging the tank, i.e., removing thermal energy from the tank, further degrades the thermocline in a thermal storage tank. One approach for minimizing thermocline degradation and improving the utilization factor is to periodically discharge the tank completely to eliminate the thermocline and then re-establish a thermocline by fully charging the tank. However, this approach is inefficient, cumbersome, and not operationally practical.
In order to more efficiently store thermal energy in a fluid contained within a thermal energy storage tank, it is desirable to maintain a region of relatively warmer fluid separate from and above a region of relatively cooler fluid. As such, it is desirable to maximize the utilization factor by minimizing the width of the thermocline region.
Accordingly, it is an objective of the present invention to provide a thermal energy storage tank for optimally managing a thermocline in a thermal energy storage fluid contained within the thermal energy storage tank.